During operation of an agitator mill, the grinding bodies in the grinding area are subject to wear so that their total quantity is gradually diminished. When fine grinding ferrites and similarly abrasive material, the wear on the grinding bodies can be so great that a certain amount of grinding bodies must be replenished in the grinding area on every day of production in order to keep about constant the part by volume of the grinding area which is filled with grinding bodies. In larger agitator mills, a daily loss of several kilograms of grinding bodies can occur.
In closed agitator mills, in particular continuously working ones, slight excess pressure prevails in the grinding area. For this reason, grinding bodies cannot easily be replenished during operation of such an agitator mill; it is therefore usual to compensate for the loss of grinding bodies only at greater intervals in time, for example, daily before starting up operation or after operations ends, by replenishing grinding bodies. However, working with a filling of grinding bodies which steadily lessens during the course of the day has as its result that material ground in the evening is less fine than material ground in the morning although the material to be ground has been in the grinding area for an identical length of time.
This disadvantage can be avoided if the agitator mill has a device which works continuously or at short intervals to feed grinding bodies from a storage vessel into the grinding area. Such a device comprising a housing disposed between storage vessel and grinding area and a drivable conveying member disposed therein is known from DE No. 2 242 174 A1. In this device, the housing has a cylindrical bore opening out into the grinding area, said bore having a slighty smaller diameter than the diameter of the grinding bodies. The wall of the bore is resilient so that the grinding bodies can be pressed through the bore one after the other and each time at least one grinding body tightly seals the bore. A piston is disposed in a portion of the bore averted from the grinding area to act as a conveying member and said piston is movable to and fro by a crank gear so that it exerts force by strokes on the hindmost grinding body and this force is transmitted over several grinding bodies lying one behind the other in the bore so that on each forward stroke the foremost grinding body is pushed into the grinding area. On each back stroke, the piston makes free the opening of a supply line through which a fresh grinding body from the storage vessel falls into the bore.
It has become apparent that this known device only works reliably if the grinding bodies are all of the same size and have a relatively exact spherical form. These conditions are only fulfilled by new and well-sorted grinding spheres; used grinding bodies cannot be fed anew by means of the known device into the grinding area.